METHOD FOR MODIFYING SURFACE OF Co-Cr ALLOY, METHOD FOR MANUFACTURING HIGH FATIGUE STRENGTH Co-Cr ALLOY, AND HIGH FATIGUE STRENGTH Co-Cr ALLOY

ABSTRACT

Provided is a method for modifying a surface of a Co-13 Cr alloy to obtain the Co-13 Cr alloy superior in fatigue strength. The method for modifying a surface of a Co-13 Cr alloy, comprising a step of shot peening of the Co-13 Cr alloy using a shot material including ZrO2.

TECHNICAL FIELD

The present disclosure relates to a method for modifying a surface of aCo-13 Cr alloy, a method for manufacturing a high fatigue strength Co-13Cr alloy, and a high fatigue strength Co-13 Cr alloy.

BACKGROUND

As an implant material, a biocompatible Co-13 Cr alloy is used. Co-13 Cralloy has higher strength than Ti, and thus is used as materials formembers undergoing high load such as pins for the spinal cord. A methodfor improving the strength by heat treatment of such a Co-13 Cr alloy isknown (for example, Patent Literature 1).

CITATION LIST Patent Literature

[Patent Literature 1] Japanese Unexamined Patent Publication No.2014-74227

SUMMARY

The implant material is required to be replaced in a timely mannerbecause of deterioration over time and the like, and currently, theliving body always undergoes a large load each time the replacement isperformed. In order to reduce the frequency of the replacement,development of an implant material that can withstand longer use isrequired.

The present disclosure has been made in view of the above circumstances,and an object of the present disclosure is to provide a method formodifying a surface of a Co-13 Cr alloy to obtain a Co-13 Cr alloysuperior in fatigue strength. The other objects of the presentdisclosure are to provide a method for manufacturing such a high fatiguestrength Co-13 Cr alloy and a high fatigue strength Co-13 Cr alloyobtained by the manufacturing method.

The present disclosure provides a method for modifying a surface of aCo-13 Cr alloy, the method comprising a step of shot peening of theCo-13 Cr alloy using a shot material including ZrO₂.

The present disclosure also provides a method for manufacturing a highfatigue strength Co-13 Cr alloy, the method comprising a step of shotpeening of a Co-13 Cr alloy using a shot material including ZrO₂.

In one aspect of the surface modification method and the manufacturingmethod of the present disclosure, the average grain size of the shotmaterial may be 0.05 to 1.0 mm.

The present disclosure also provides a high fatigue strength Co-13 Cralloy in which the ε-hcp phase gradually increases and the γ-fcc phasegradually decreases from the inside to the surface.

In one aspect of the alloy of the present disclosure, the Vickershardness (HV) of the surface may be 500 or more.

The present disclosure can provide a method for modifying a surface of aCo-13 Cr alloy to obtain the Co-13 Cr alloy superior in fatiguestrength. The present disclosure also can provide a method formanufacturing such a high fatigue strength Co-13 Cr alloy and a highfatigue strength Co-13 Cr alloy obtained by the manufacturing method.

The present disclosure enables the production of an implant that canhave high fatigue strength and a long life. The surface modification byshot peening does not change the component of the material and thus itis considered that the surface-modified material can easily meet theacceptance criteria as an implant material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing surface roughness Rz of the test materialbefore and after shot peening.

FIG. 2 is a graph showing XRD diffraction results of the test materialbefore and after shot peening by GB-K.

FIG. 3 is a graph showing the proportion of theε phase in the testmaterial after shot peening.

FIG. 4 is a photograph showing EBSD observation results of the testmaterial before and after shot peening.

FIG. 5 is a graph showing the Vickers hardness (HV) of surface andrelationship of distance from surface and Vickers hardness of the testmaterial before and after shot peening.

DETAILED. DESCRIPTION

The best mode for carrying out the present disclosure will be describedbelow.

[Method for Modifying Surface of Co-13 Cr Alloy]

A method for modifying a surface of a Co-13 Cr alloy of the presentembodiment comprises a step of shot peening of a Co-13 Cr alloy using ashot material including ZrO₂.

The surface modification method of the present embodiment is to modifythe alloy surface by utilizing phase transformation (stress-inducedmartensitic transformation from γ-fcc phase to ε-hcp phase) of a Co-13Cr alloy. It is considered that using a zirconia material mainlycomposed of low thermal conductivity ZrO₂ as a shot material canefficiently provide the thermal energy of shot peening to the alloyside, and hence the treatment equivalent to heat treatment can belocally and instantaneously applied to the alloy. According to thesurface modification method of the present embodiment, selective phasetransformation in the vicinity of the surface of the Co-13 Cr alloy intothe ε phase can realize a material in which the internal toughness ismaintained while the vicinity of the surface is high in strength(gradient material). Such a material having strength against externalload and also having flexibility is a material excellent in fatiguestrength (high fatigue strength material).

As the Co—Cr alloy, a Co—Cr—Mo alloy can be used, and in particular, aCo—28Cr—6Mo alloy in accordance with ASTM F1537 can be used. ThisCo—28Cr—6Mo alloy includes Co as a main component, and the Cr content is26.3 to 3 0.0% by mass, the Mo content is 5.0 to 7.0% by mass, and Ni,Mn, Si, C, Fe, N, and the like as other components are included in traceamounts. Such Co-13 Cr alloys are biocompatible and can be used inmedical applications such as implants.

The shot material includes ZrO₂. Since the zirconia material has lowthermal conductivity, it easily causes phase transformation in thevicinity of the alloy surface. From the viewpoint of suppressing thetransfer of thermal energy generated in the shot peening step to theshot material side, the content of ZrO₂ in the shot material may be 30%by mass or more, 60% by mass or more, or 100% by mass (the shot materialsubstantially consisting of ZrO₂). The shot material may include a smallamount of a compound including Fe, Cr, Si, Al, Cu or the like as aconstituent element (for example, Al₂O₃ or SiO₂) as a component otherthan ZrO₂.

The average grain size of the shot material may be 0.05 to 1.0 mm, 0.1to 0.75 mm, 0.1 to 0.5 mm, 0.1 to 0.3 mm or 0.1 to 0.15 mm. An averagegrain size of less than 0.05 mm causes the shot material to be light andthus sufficiently modifying the Co-13 Cr alloy surface tends to bedifficult, while an average grain size of more than 1.0 mm leads to alonger time for achieving the desired visual coverage (the ratio of thearea occupied by the shot material dents by visual observation) and thusthe treatment efficiency tends to deteriorate. The average grain sizehere is a value measured using a sieve.

The Vickers hardness (HV) of the shot material may be 500 to 1300, 600to 1200, 700 to 1100, and 900 to 1100. The Vickers hardness of less than500 is difficult to cause phase transformation to a sufficient depth,while the Vickers hardness of more than 1200 causes the surfaceroughness to be too large, tending to easily occur crack fracture.

The density of the shot material may be 1 to 10 g/cm³, 2.5 to 7.5 g/cm³,and 4 to 6 g/cm³. The density of the shot material of less than 1 g/cm³leads to the weak intensity, tending to be difficult to cause phasetransformation to a sufficient depth, while the density of the shotmaterial of more than 10 g/cm³ leads to the strong intensity, causingthe surface roughness to be larger to tend to easily occur crackfracture.

Examples of the method of the shot peening include a rotary projectionmethod, an air suction method, and a pressure blast method. Among them,the air suction method can be used from the viewpoint that theconstruction of a complicated apparatus is unnecessary. The air suctionmethod is also advantageous in that it is easy to suppress thedestruction of the shot material itself including ZrO₂.

The injection pressure may be set appropriately, but can be, forexample, about 0.1 to 0.5 MPa. The injection pressure of less than 0.1MPa causes the shot material to tend to be difficult to eject, while theinjection pressure of more than 0.5 MPa even causes the hardness aftermodification to tend to saturate.

The injection time can be set appropriately to achieve the desiredcoverage. The coverage may be at least 200% or more, 500% or more, 750%or more, or 900% or more. The upper limit of the coverage can be set to1000% from the viewpoint of sufficient proceeding of the phasetransformation in the vicinity of the surface of the Co-13 Cr alloy. Thestate where all regions are covered by the dents after striking the shotmaterial on the surface is referred to as coverage of 100%. For example,coverage of 200% means that the shot is performed only for the timerequired to further reach coverage 0% to 100% from the state of coverage100%.

The surface roughness (Rz) of the Co-13 Cr alloy after the shot peeningmay be less than 10 μm or less than 5 μm. For example, when the use of aCo-13 Cr alloy in an artificial joint or the like is assumed, thepolishing step is performed after the shot peening as described belowfor the purpose of suppressing the generation of wear powder on thesliding surface. When the surface roughness after the shot peening is 10μm or more, the amount of polishing increases until the mirror surfaceis achieved. This is not only inefficient in the process but also leadsto an excessive removal of the layer modified by the shot peening.

The surface roughness of the alloy and the amount of phasetransformation in the vicinity of the alloy surface can be adjusted bychanging various conditions of the shot peening (type of the shotmaterial, injection pressure of the shot material, the amount ofcoverage, and the like). Since this can adjust the tensile strength,hardness, and the like of an alloy, a desired mechanical property can begiven to the alloy.

The modification of the alloy surface by the shot peening (theintroduction of a high strength phase on the surface) can beconveniently determined by measuring the Vickers hardness (HV) of thealloy surface.

[Method for Manufacturing High Fatigue Strength Co-13 Cr Alloy]

The method for manufacturing a high fatigue strength Co-13 Cr alloyaccording to the present embodiment comprises a step of shot peening ofa Co-13 Cr alloy using a shot material including ZrO₂. Various types ofmaterials, conditions, and the like regarding the step of shot peeningconform to the contents of the method for modifying the surface of theCo-13 Cr alloy.

The manufacturing method of the present embodiment may further comprisea polishing step after the step of shot peening. That is, theshot-peened Co-13 Cr alloy surface may be further polished. Thepolishing step can be performed by, for example, mechanical polishing,electrolytic polishing, chemical polishing, or the like.

[High Fatigue Strength Co-13 Cr Alloy]

The high fatigue strength Co-13 Cr alloy of the present embodiment is analloy in which the ε-hcp phase gradually increases and the γ-fcc phasegradually decreases from the inside to the surface. Such characteristicmaterials (gradient materials) can be introduced by the shot peeningwhich can selectively cause phase transformation only in the vicinity ofthe surface. That is, the high fatigue strength Co-13 Cr alloy of thepresent embodiment can be a Co-13 Cr alloy which is subjected to shotpeening (preferably using a shot material including ZrO₂). Moremacroscopically, the high fatigue strength Co-13 Cr alloy of the presentembodiment can also be an alloy including an inner layer substantiallyconsisting of a γ-fcc phase and an outer layer having an ε-hcp phase.

The Vickers hardness (HV) of the surface of the high fatigue strengthCo-13 Cr alloy of the present embodiment may be 500 or more, 550 ormore, or 600 or more. In view of the fact that the Vickers hardness ofthe surface of the Co-13 Cr alloy before the shot peening is about 400,the surface strength is significantly improved.

The effect of the shot peening reaches a certain depth from the alloysurface. Though not generally speaking because of the dependence ontreatment conditions, for example, in the region of less than 400 μm, orless than 350 μm, or less than 300 μm from the surface, the Vickershardness of the alloy is improved as compared to the untreated case.This is considered to be because the phase transformation from the γ-fccphase (the phase having the existing toughness) to the ε-hcp phase (thephase having the high strength) occurs to such a depth. That is, in thehigh fatigue strength Co-13 Cr alloy of the present embodiment, theouter layer having the ε-hcp phase is formed with a thickness of lessthan 400 μm on the inner layer consisting of the γ-fcc phase.

The analysis by the XRD diffraction method can quantitatively calculatethe proportion (volume ratio) of the formed ε-hcp phase. In addition,the analysis by the EBSD method can evaluate the proportion of the ε-hcpphase in consideration of the internal direction. In the high fatiguestrength Co-13 Cr alloy, the proportion of the ε-hcp phase in thevicinity of the surface (a depth of about 15 μm from the surface) may be30% or more, 35% or more, or 40% or more. Using a shot materialincluding ZrO₂ easily allows for the proportion of the ε-hcp phase to be40% or more. The remainder is substantially the γ-fcc phase. In view ofthe fact that the proportion of the ε-hcp phase before the shot peeningis about 0%, a considerable amount of the ε-hcp phase is introduced.Increasing the coverage also can further increase the proportion of theε-hcp phase.

Crystal grains in the vicinity of the alloy surface are refined by theshot peening. In the high fatigue strength Co-13 Cr alloy, the averagesize of the crystal grains may be less than 3 μm, less than 2 μm, lessthan 1.5 μm, or less than 1 μm. In view of the fact that the averagesize of the crystal grains before the shot peening is about 6 μm, thecrystal grains are significantly refined.

The existence mode of the γ-fcc phase and the ε-hcp phase in the Co-13Cr alloy and the existence mode of the refined crystal grains can beobserved by, for example, EBSD (Electron Back Scatter DiffractionPatterns) method.

The high fatigue strength Co-13 Cr alloy of this embodiment is suitableas an implant material, and the surface roughness and hardness can beadjusted, so that it can be widely used for sliding parts, pins for thespinal cord, and the like.

EXAMPLES

Hereinafter, the present disclosure will be more specifically describedreferring to Examples. However, the present invention is not limited tothese Examples.

(Experiment 1: Comparison of Material of Shot Material)

As a test material, a medical Co—28Cr—6Mo alloy (ASTM F1537) was used.Table 1 shows the alloy composition (unit: % by mass). In the shotpeening, the surface to be treated was polished in advance with Al₂O₃powder and used.

TABLE 1 Co Cr Mo Ni Mn Si Fe N C Bal. 26.3- 5.0- ≤1.0 ≤1.0 ≤1.0 ≤0.75≤0.25 ≤0.14 30.0 7.0

Under the shot peening conditions shown in Table 2, the test materialwas subjected to the shot peening with a nozzle size of 6 mm, aninjection distance of 100 mm, and a coverage of 300%. The apparatus ofthe air suction method was used for the shot peening.

The surface roughness Rz of the test material before and after the shotpeening was measured using a contact-type surface roughness measuringdevice. The measurement results are shown in FIG. 1. In the figure, NPis data of an untreated test material (only subjected to Al₂O₃polishing) without the shot peening.

The test material before and after the shot peening was analyzed by XRDdiffraction using an X-ray diffractometer. The XRD diffraction resultsof the test material before and after the shot peening are shown in FIG.2. From the main peak in the XRD chart, the proportion of ε phase wascalculated using the following formula. In the following formula, Vrepresents a volume fraction and I represents an integrated intensity ofa peak.

$\begin{matrix}{V_{ɛ} = \frac{{I_{ɛ}\left( {10\overset{\_}{1}0} \right)} + {I_{ɛ}\left( {10\overset{\_}{1}1} \right)}}{{I_{ɛ}\left( {10\overset{\_}{1}0} \right)} + {I_{ɛ}\left( {10\overset{\_}{1}1} \right)} + {I_{\gamma}(111)}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

The proportion (volume ratio) of the ε phase in the test material afterthe shot peening calculated above is shown in FIG. 3. From the figure,it is found that the shot material having the largest transformationratio to the ε phase is GB-K. From this, it is considered that the shotmaterial having a small grain size had a high frequency of collisionwith the projection surface, and as a result, a large strain was formed.Comparison of SBM44T with ZB120 finds that the proportion of ε phase inZB120 with high hardness is larger. From this, it is considered that thecollision energy is larger as the hardness of the shot material ishigher. As described above, it is considered that a shot material havinga small grain size and high hardness is suitable to promote thestress-induced martensitic transformation.

The test material before and after the shot peening was then analyzed bythe EBSD method using a field emission scanning electron microscope.FIG. 4 shows EBSD observation results of the test material before andafter the shot peening. The upper drawing in FIG. 4 shows the existencemodes of the γ phase and the ε phase in the vicinity of the surface ofthe test material, and the lower drawing shows the appearance of crystalgrains in the vicinity of the surface of the test material. The ε phaseproportion in the upper drawing was obtained from the observation imageby analysis software. GS in the lower drawing means an average grainsize. After taking out the band contrast from the observation image bythe analysis software, the grain boundary line was drawn, the partsurrounded by the line was made into one grain boundary, and the averagegrain size of the obtained crystal was calculated. As a result ofobservation by EBSD, it was found that the shot peening introduced the εphase and that the crystal grains in the vicinity of the surface wererefined. For GB-K (refer to FIG. 3) in which the amount oftransformation to the ε phase was the largest, phase transformation tothe ε phase mainly occurred in the vicinity of the surface, and manyuntransformed γ phases were present inside. From this, it is consideredthat the collision energy is larger as the hardness of the shot materialis higher, and as a result, the stress-induced martensitictransformation occurred to the inside.

FIG. 5 shows the Vickers hardness (HV) of the test material before andafter the shot peening.

TABLE 2 Shot material GB-K GB-D SBM100C SBM44T RCW06PS ST600 ZB120Material Glass Iron-based WC ZrO₂ Grain size (mm) 0.09 0.3 0.15 0.1 0.60.12 Hardness (HV) 550 830 750 1450 700 Injection pressure 0.3 0.2 0.3(MPa) Density (g/cm³) 2.3 7.9 15.6 5.7 ε phase proportion 33.4 27.5 15.05.5 40.5 41.6 46.0 (%)

(Experiment 2: Comparison of Type of Zirconia Shot Material)

The shot peening was performed on the test material in the same manneras in Experiment 1 except that the shot peening conditions were changedas shown in Table 3. All shot materials A to E include ZrO₂ as a maincomponent. In the table, shot materials A, C, D, and E are manufacturedby Saint-Gobain company, and shot material B is manufactured by TosohCorporation.

TABLE 3 Shot material A B C D E B120 TZ-B90 B60 B30 B205 MaterialCeramic (main component ZrO₂) Grain size (mm) 0.1 0.12 0.2 0.6 0.05Hardness (HV) 700 1085 700 Injection pressure (MPa) 0.3 Density (g/cm³)3.8 6.05 3.8 3.8 3.8 Coverage (%) 300 or 1000

In the same manner as Experiment 1, the proportion of the ε phase afterthe shot peening under each condition was calculated from theobservation results of EBSD. The results are shown in Table 4.

TABLE 4 Shot material Coverage ε phase proportion NP — 1.0% A  300%42.1% 1000% 64.4% B  300% 37.7% 1000% 74.9% C  300% 34.5% 1000% 64.8% D 300% 44.7% 1000% 63.4% E  300% 32.9% 1000% 35.8%

According to Examples, the gradient material was formed by selectivephase transformation (stress-induced martensitic transformation) in thevicinity of the surface of the Co-13 Cr alloy into the ε phase, andthereby it was possible to obtain a material in which the internaltoughness was maintained while the vicinity of the surface was high instrength. That is, the material excellent in fatigue strength (highfatigue strength material) having strength against external load andalso having flexibility was able to be obtained.

What is claimed is:
 1. A method for modifying a surface of a Co-13 Cralloy, comprising a step of shot peening of the Co-13 Cr alloy using ashot material including ZrO₂.
 2. The method for modifying a surface of aCo-13 Cr alloy according to claim 1, wherein an average grain size ofthe shot material is 0.05 to 1.0 mm.
 3. A method for manufacturing ahigh fatigue strength Co-13 Cr alloy, comprising a step of shot peeningof a Co-13 Cr alloy using a shot material including ZrO₂.
 4. The methodfor manufacturing a high fatigue strength Co-13 Cr alloy according toclaim 3, wherein an average grain size of the shot material is 0.05 to1.0 mm.
 5. A high fatigue strength Co-13 Cr alloy, wherein an ε-hcpphase gradually increases and a γ-fcc phase gradually decreases frominside to surface.
 6. The Co-13 Cr alloy according to claim 5, wherein aVickers hardness of the surface is 500 or more.